1. Field of the Invention
This invention relates to a camera which produces image signals.
2. Description of the Prior Art
In a camera which generates image signals (a so-called video camera), it has heretofore been commonly practised to use an image pick-up tube as image signal generating means and to use a picture tube as image signal monitor means (finder). Both of the image pick-up tube and the picture tube are operated by emission of thermal electrons from the cathodes thereof and therefore, a considerably long time, for example, of the order of ten seconds, has been required for the respective cathodes to reach a predetermined operating temperature. Accordingly, in the case of photographing, if the heating of the cathodes is started by a heater, the photographing speed is lost and for this reason, it has heretofore been practised in the image recording preparatory condition to set the cathodes of the image pick-up tube and the picture tube to their heated condition by setting the image recording button of a video tape recorder for recording image signals connected to the camera by a cord. This has led to a great power consumption in the image recording preparatory condition which in turn has led to a difficulty in respect of the service life of the battery in the camera video tape recorder system loaded with a battery.
Also, in the conventional camera of this type, a finder provided only by such an optical member as is used in 8 mm cameras, instead of a picture tube, is sometimes used as the means for confirming the photographic view field, for the purpose of reducing the cost of the camera. In such case, as compared with the case where the conventional picture tube is used, the confirmation of whether or not the image pick-up tube has reached its normal operative condition cannot completely be effected and this has given rise to a danger that image recording is effected by mistake even when the picture signal output level is not proper. Particularly, as described above, mcuh time is required for the cathodes to reach their normal operating temperature when photography is started and therefore, particularly in a case where a fast photographing speed is required, it has often happened that improper image recording is effected.
Further, the image recording has heretofore been accomplished by converting an image into an electrical signal by an image pick-up tube and causing a recording apparatus to record the image signal.
As such image pick-up tube, as is well-known, means such as vidicon, planvicon, silicon vidicon, calnicon, saticon, newvicon, etc., have been developed and improved along with the technical advance and nowadays, for domestic use and public welfare, such means are being endowed with a performace sufficiently satisfactory in resolving power and color reproducibility. However, the image pick-up tube has a disadvantage that a picture signal is not generated in five to ten seconds after power has been applied thereto, namely, until the heater of the image pick-up tube is sufficiently heated. Accordingly, where an electronic finder is used as the finder, namely, where the output of the image pick-up tube is applied to the picture tube to visualize the image, if the image is obtained while the photographer is looking into the electronic finder, the image recording can be started by camera trigger. However, in a camera of the optical finder type, an optical image is obtained in the finder independently of the temperature of the heater of the image pick-up tube and this leads to a disadvantage that before the heater of the image pick-up tube is sufficiently heated, the camera is triggered to start the image recording and when the video tape (hereinafter referred to as the tape) is played back, it is found that no image has been recorded thereon for five to ten seconds. The camera of the optical finder type, as compared with the camera of the electronic finder type, has the above-described disadvantage, but it has merits such as light weight, low cost, compactness, small consumed power, etc., and it is best suited for the outdoor image recording using a battery.